The invention relates to a capacitive sensor circuit comprising: a sensor electrode forming a first plate of a sensing capacitor; a first inverting amplifier, having an input connected to the sensor electrode and having an output; a second amplifier, having an input coupled to the output of the first inverting amplifier and having an output; a feedback resistor connected between the output and the input of the first inverting amplifier.
Such a sensor circuit is known from European Patent Application No. EP 0,524,708, where the circuit is used to turn on a personal-care apparatus when it is lifted by the user of the apparatus and to turn off the apparatus when the apparatus is put down. The sensor electrode, the first inverting amplifier and the feedback resistor form an oscillator, also referred to as an astable multivibrator, whose oscillation frequency is dictated inter alia by the value of the feedback resistor and by the magnitude of the capacitance between the sensor electrode and the signal earth terminal of the capacitive sensor circuit. The oscillation frequency decreases as the capacitance increases. When the user picks up the apparatus the capacitance relative to earth is comparatively high and the frequency is comparatively low. If the apparatus has been put down the capacitance relative to earth is comparatively low and the frequency is comparatively high. The frequency difference or the frequency ratio between the comparatively low and the comparatively high oscillator frequency is measured by means of a frequency-to-voltage converter and is converted into a voltage difference, which energizes a switch to turn on the apparatus. The second amplifier then serves as a buffer between the oscillating first inverting amplifier and the frequency-to-voltage converter. This known capacitive sensor circuit is fairly susceptible to spreads in properties and values of the components used. It is known, inter alia from European Patent Specification EP 0,104,422, to compare the oscillation frequency with that of a second oscillator which comprises a reference capacitance relative to earth. The capacitance of the sensor electrode is compared with the capacitance of the reference capacitor by means of a frequency measurement. A gate circuit provides a signal which is a measure of the oscillation frequencies. This known solution can be accurate but requires the use of a gate circuit or some other coincidence circuit.